Seeger's best known songs include "Where Have All the Flowers Gone," "Turn! Turn! Turn! (To Everything There Is a Season)" and "If I Had a Hammer (The Hammer Song)."

But his influence extended far beyond individual hits.

His grandson Kitama Cahill Jackson told CNN that the singer died of natural causes at New York Presbyterian Hospital on Monday evening.

Familiar with controversy

In a career spanning more than 70 years, Seeger frequently courted controversy.

"He lived at a time when so many things hadn't been done yet, the idea of making music about something hadn't really been done," Jackson said. "And now people do it all the time."

Seeger's opinions didn't always sit well with authorities.

"From the start, he aspired to use folk music to promote his left-wing political views, and in times of national turmoil that brought him into direct confrontation with the U.S. government, corporate interests, and people who did not share his beliefs," William Ruhlmann wrote in a biography on allmusic.com. "These conflicts shaped his career."

Early career

In 2009, Seeger talked to CNN about the beginnings of his music career in the late 1930s.

"I come from a family of teachers, and I was looking for a job on a newspaper and not getting one," he said in the interview. "I had an aunt who said, 'Peter, I can get five dollars for you if you come and sing some of your songs in my class.' Five dollars? In 1939, you would have to work all day or two days to make five dollars. It seemed like stealing."

But Seeger said he took his aunt up on the offer.

"Pretty soon I was playing school after school, and I never did work on a newspaper," he said "You don't have to play at nightclubs, you don't have to play on TV, just go from college to college to college, and the kids will sing along with you."

Last days

Jackson, Seeger's grandson, said the singer-songwriter had heart surgery in December to replace a valve, which had gone well and had nothing to do with his death.

He said Seeger was in the hospital for six days before his death.

He couldn't speak for the last three days, Jackson said, but his mind never went away and he continued to recognize people.

"He was a second father to me, he was a friend, he was a best friend," Jackson said. "He was just this wonderful genuine person."